A spark plug is used in an internal combustion engine for ignition of a combustible gas. In general, the spark plug has an insulator formed with an axial hole, a center electrode disposed in a front end side of the axial hole and a terminal electrode disposed in a rear end side of the axial hole. In order to maintain a certain clearance between the insulator and the other plug components such as center electrode or terminal electrode, it is required to produce the insulator within certain tolerances of circular runout. An inspection test is thus conducted in the process of production of the insulator, so as to check whether the insulator is within the certain tolerances of circular runout.
For example, Japanese Laid-Open Patent Publication No. H11-242979 discloses a technique of measuring a circular runout of a spark plug insulator by holding a front part of the insulator with a chuck, rotating the insulator about its axis and bringing a dial gauge into contact with a rear part of the insulator.
In the above-disclosed circular runout measurement technique, however, there is a possibility that the both of the front and rear parts of the insulator may sustain damage or dirt by contact with the chuck and the dial gauge. The damage or dirt on the insulator becomes a cause of problems such as through discharge and creep discharge. In addition, there has recently a demand to reduce the diameter of the spark plug. The diameter reduction of the spark plug leads to a high need for precise control of the circular runout of the front part (in particular, a leg portion) of the insulator so as to ensure the clearance between the insulator and the center electrode. It is accordingly required to protect the leg portion of the insulator from damage or dirt during the measurement of the circular runout.
In view of the foregoing, the present invention provides a technique of inspecting the circular runout of a front part (including a leg portion) of a spark plug insulator while protecting the leg portion of the insulator from damage and dirt.